This project concerns the sequence of events that occurs in mating mixtures and leads to the formation of zygotes in the yeast Sacchromyces cerevisiae. We are particularly interested in the hormone-like extracellular sex factors, called alpha-factor and a-factor, that are produced by cells of one mating type and cause a variety of effects in cells of the opposite mating type. Our approach to the study of these events is primarily genetic, but also involves detailed physiological and biochemical analysis of mutants. We have isolated mutants by several different criteria, corresponding to different aspects of the conjugation process--ability to mate, ability to respond to sex-factor, ability to produce sex-factor. In addition to the phenotypic characteristics used to select mutants, there are a variety of other phenotypic anomalies that can be observed in these mutants--failure to agglutinate, failure to destroy sex-factor, abnormal morphology, failure to sporulate. One can infer from these phenotypes that there are relationships - regulatory, structural, functional - among the particular processes that underlie these phenotypes, and that they are associated with the biological function called sexual conjugation. Our objective is to identify these underlying processes, and to understand the relationships among them.